1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an external optical modulator. Although the present invention is suitable for a wide scope of application, it is particularly suitable for simultaneously modulating two independent optical channels in one modulator.
2. Discussion of the Related Art
An external optical modulator (EOM) is an important component in wavelength division multiplexed (WDM) optical transmission systems. In such systems, an electrical signal representing the information to be transmitted is applied to the EOM, which then modulates a continuous wave (CW) laser beam that propagates the modulated light through the transmission system. One of the advantages of external optical modulation over directly modulating the laser is that data can be transmitted relatively chirp-free, thereby reducing errors in long distance transmission. Chirp is an instantaneous change in optical frequency, which accompanies the process of directly modulating the laser diode. Chirp interacts with the dispersion profile of the transmission fiber to severely limit the distance over which error-free data transmission is possible. External optical modulators can be designed with little or no chirp, enabling a much higher transmission distance than can be obtained with direct modulation.
In a conventional optical modulator, the dual electrode design concept was developed so that the data and its complement can be separately applied to the two electrodes of the dual electrode modulator. This is also known as differential driving. By adjusting the relative amplitudes of the data and its complement the chirp parameter of the modulator can be fixed to any desired value between -.varies. and +.varies..
Conventionally, externally modulated WDM transmission systems have employed one EOM per optical channel. Further, the EOM is one of the more expensive optical components employed in the system. It would therefore be advantageous to provide an apparatus wherein two or more optical channels can be modulated with one EOM.